The present invention relates to novel 1,3,4-thiadiazole derivatives having useful antiviral activity against herpes virus, cytomegalovirus (CMV). The invention also relates to a pharmaceutical composition containing such compounds and methods for the treatment of such viral infections. Many of these compounds are also active against other herpes viruses, such as the varicella zoster virus, the Epstein-Barr virus, the herpes simplex virus, and the human herpes virus type 8 (HHV-8).